My article “Intellectual Property and Libertarianism” [Liberty‘s online version; local PDF] was published last month in Liberty magazine (December 2009 issue). This article is based in part on a speech at Mises University 2009 (July 30, 2009; audio; video, speech podcast on The Lew Rockwell Show, #131, as The Intellectual Property Racket (Aug. 19, 2009)) and also on my What Libertarianism Is, which contains references not present in the Liberty paper (as does The Case Against IP: A Concise Guide). A version with endnotes appears here in Mises Daily.
Update: See also Yeager and Other Letters Re Liberty article “Intellectual Property and Libertarianism”, and KOL013 | “Intellectual Property and Libertarianism,” Mises University 2009.